Anisotropic Rotation of TEMPO Radical in the Cavities of Metal-Organic Framework ZIF-8 Induced by Guest Solvents

Nitroxide radicals, encapsulated into the cavities of metal-organic frameworks (MOFs), have recently drawn attention as multifunctional spin probes. In this work, we investigate the mobility of TEMPO nitroxide in ZIF-8 framework in the presence of various solvent molecules located in the same cavity. We employ simulations of continuous wave (CW) EPR spectra to reveal anisotropic rotation of radical. It is shown that the largest anisotropy of rotation occurs for ZIF-8 impregnated with alcohols: it is most pronounced at low temperatures, but also manifested at room temperatures. Linear alkanes do not indicate any significant effect onto the rotation anisotropy of TEMPO, while various benzene derivatives show more complicated trends caused by their bulkier shapes. With a number of examples, we demonstrate the ability of CW EPR analysis to disclose effects of solvent ordering in MOFs via anisotropic rotation of encapsulated spin probe.